In the electronics industry, conductive wire is used in a variety of devices, such as semiconductor devices for example, to connect portions of the device. Exemplary materials for wire bonding are gold, aluminum, copper and silver. A wire bond is formed by attaching a length of wire between two contact/bonding locations (e.g., a first location being on a semiconductor die, and the second location being on a substrate supporting the die). In order to form the attachment, various devices are used to sever and bond the wire ends to the contact locations. Known wire bonding apparatuses include thermocompression (T/C), thermosonic (T/S) or ultrasonic (U/S) devices. The resulting length of bonded wire is typically curved along its length in a generally parabolic or elliptical configuration and is, therefore, referred to as a wire “loop”.
Two known techniques for bonding a wire to a semiconductor device are ball bonding and wedge bonding. Ball bonding is generally the preferred technique, particularly in the semiconductor industry in which more than 90 percent of all semiconductor devices are manufactured using ball bonding.
Referring to FIGS. 1A through 1G, there is shown schematically a process of forming a wire loop on a substrate using a known ball bonding technique. The exemplary bonding apparatus includes capillary 10 carried by a bond head (not shown) for movement with respect to substrate 12. In FIGS. 1A through 1G, capillary 10 is illustrated as moving as indicated by the arrows. Although the bonding process is described herein as being accomplished by moving capillary 10, it should be understood that the desired relative movement between capillary 10 and substrate 12 could also be achieved by moving substrate 12, or by a combined movement of both capillary 10 and substrate 12. Capillary 10 defines a central passage in which fine wire 14 is received such that wire 14 is fed from working tip 16 of capillary 10.
The bonding apparatus also includes clamp 18, which is carried by the bond head adjacent capillary 10 for preventing relative movement between capillary 10 and wire 14. Referring to FIG. 1A, a terminal end portion of wire 14 extending beyond working tip 16 of capillary 10 is formed into substantially spherical ball 20. Ball bonding apparatuses typically include an electronic flame-off (EFO) wand (not shown) that, when fired, generates a spark that melts the extending end portion of the wire. As the molten end portion of the wire solidifies, surface tension forms the end portion into a substantially spherical configuration, which is sometimes referred to in the art as a “free-air ball.” Ball 20 is bonded to substrate 12 (i.e., a bonding location of substrate 12 such as a contact pad of substrate 12) by the bonding apparatus as described below.
As shown in FIG. 1B, with clamp 18 open, capillary 10 is directed downwardly until ball 20 touches contact 22 on substrate 12. Compressive force is then applied to ball 20 by working tip 16 of capillary 10 while heat and vibration is applied by the bonding apparatus. The combined effects of the compressive force, heat and vibration flattens the ball 20 and bonds ball 20 to contact 22 through interfacial interaction between ball 20 and contact 22.
Referring to FIG. 1C, capillary 10 is then moved vertically away from contact 22 with clamp 18 in an opened condition such that wire 14 is fed from working tip 16 of capillary 10. As shown in FIG. 1D, a horizontal component is then added to the movement of capillary 10 to direct capillary 10, and wire 14 is fed from capillary 10, to a second bond location on substrate 12. Capillary 10 is then moved into contact with substrate 12 at the second bond location where compressive force, heat and vibration is again applied to bond wire 14 to substrate 12 at the second location, thus forming wire loop 24.
Referring to FIGS. 1F and 1G, capillary 10 is then moved by the bonding apparatus a short distance away from the second location on substrate 12 to feed a terminal end portion of wire 14 from capillary 10, which will form the free-air ball for the next wire loop. With clamp 18 closed, capillary 10 is then moved further away from substrate 12 such that wire 14 is separated from substrate 12 as shown in FIG. 1G.
Referring to FIG. 2, completed wire loop 24 is shown in greater detail. As shown, the above-described bonding process results in a flattening of ball 20 at the first bond location on contact 22. The capillary of ball bonding apparatuses typically includes chamfered surfaces at the working tip such that substantially conical portion 26 of ball 20 is formed. As described above, capillary 10 of the prior ball bonding apparatus is moved vertically into contact with the first bond location. The movement of the capillary remains vertical (i.e., perpendicular to the bond plane at first contact location) as ball 20 is bonded to contact 22. As a result, wire 14 of wire loop 24 is located at the top of the ball 20 and extends, initially, upwardly from the top of the ball 20.
Ball bonding apparatuses are also used to form flat-top bumps, also known as “flip-chip connections.” Similar to wire bonding, a ball is formed at the end of the wire and the capillary is moved along a vertical pathway to flatten the ball against a contact surface while heat and vibration are applied. Unlike a wire looping process, however, the capillary is raised from the bonded ball with the clamp closed to separate the wire from the ball. The separation results in a peaked surface at the top of the bonded ball that includes the junction between the ball and the wire. The peaked surface is then subjecting to a “coining” process in which force is applied to flatten the peaked surface to create a smooth top.
It would be desirable to provide a ball bonding process for forming a wire loop having a reduced height compared to wire loops formed by prior ball bonding processes and to provide for a desirably shaped bump without the use of a coining process.